Current methods of measuring the torque applied to an automotive column shaft are of the compliant kind and are typically accomplished by use of a torsion bar as part of the shaft, joining an upper and lower section thereof. The torsion bar is made of material with known mechanical properties and hence has known compliance. Thus, the applied torque can be calculated from a measured angular displacement, Δθ, of the torsion bar (usually in the range of plus or minus a few degrees). The calculated torque is applied to a controller which then directs an electric steering torque assist motor to provide assist torque to the column shaft.
The torque sensor device may be used to accurately measure the input torque acting on a steering column shaft in an electronic power steering (EPS) system or steer-by-wire system of a vehicle. In this application, an input torque acts on the steering column shaft when an operator turns the steering wheel. The steering column shaft includes a primary shaft and a secondary shaft. The primary and secondary shafts are connected by a torsion bar. The rotation of the primary shaft relative to the secondary shaft may be measured with a potentiometer.
Typically in the assembly of such a torque sensor device the primary shaft is operably connected to a first substrate and the secondary shaft is connected to a second substrate. The primary and secondary shafts are operably coupled by a torsion bar. Each of the first and second substrates are aligned with each other using a pin extending through an aperture in a housing containing the sensor substrates to maintain alignment while assembly with the respective primary and secondary shaft assembly. The alignment pin prevents rotation of the substrates with respect to each other and with respect to the housing during assembly. During assembly of the torque sensor, the aperture in the housing is aligned with the openings in each of the substrates to receive the alignment pin. After assembly and installation of the torque sensor to the steering column shaft, the alignment pin is slidably removed from the respective parts in an attempt to have a sensor offset after such assembly of 50 percent plus or minus 4.5 percent of the source voltage, for example Vcc (2.50 V+/−0.225V) assuming Vcc=5V.
However, due to dimensional stack up tolerances during assembly of the first and second PCB's within the housing having the alignment pin extending through each, when the pin is removed from each corresponding aperture, the required offset voltage requirement between the PCB's may be defeated by removal of the pin to allow rotation of both first and second PCB's with respect to the housing in which they are contained. The required offset is defeated because the alignment pin preventing rotation of the components also stores mechanical energy therein caused by the misalignment as a result of the stack up condition. When the pin is removed, the stored mechanical energy in the pin is reflected in rotation of the now unrestricted components, thus defeating the offset. It has been found that any deviation from the preferred offset of zero degrees greater than 0.9 degrees of the system components will cause a failure of the offset voltage requirement based on a sensor having a sensor resolution of 0.25 Volts/degree, for example.
Thus, it is desired to provide a torque sensor that will be more forgiving of assembly tolerance stack conditions, such that a required offset voltage is not affected when the alignment pin is removed after assembly. It is advantageous to provide a simplified torque sensor assembly for direct sensing of the torque applied to a shaft to which the sensor is connected. In particular it is desirable to provide a torque sensor that will accommodate larger stack tolerances when the first and second torque sensing substrates are assembled within the housing and the alignment pin is installed to prevent rotation and set the required voltage offset, such that when the pin is removed, the offset isn't affected by the mechanical energy stored in the pin as a result of the tolerance stack between the sensor and housing.